When a direct current potential is applied across an emulsion or solution containing charged molecules, the molecules migrate toward the electrode bearing the opposite charge. This phenomenon is generally called electrophoresis, and is utilized to apply coatings onto metallic surfaces for purposes of electrical insulation, paint priming, weather protection, and the like.
Most of the polymer electrophoretic deposition techniques followed in industry involve aqueous systems. However, aqueous depositions are in general markedly affected by the evolution of gases at the electrodes. This gas evolution, deriving from water electrolysis, can result in heavily pitted polymer coatings, which makes them particularly unsuitable for electrical insulation. Moreover, water emulsion systems are generally plagued by difficult-to-control surface tension and pH conditions, and viscosity difficulties.
Polyimide resins have recently come into use as high temperature electrical insulating films. Polyimide films are generally produced by film casting or dip coating of a substrate in a non-aqueous solvent solution followed by a heat cure. A major problem with the solution dip coating or casting methods is that only relatively thin films (0.001 inch for foil coatings) can be produced in a single coat. Difficulty is also encountered in uniformly coating corners, sharp edges and irregular shaped objects by the solution dip coating or casting methods because of surface tension effects.
As a partial solution, electrophoretic deposition techniques have been developed for polyamic acids in water emulsion systems, as described in U.S. Pat. No. 3,537,970. However, such an aqueous polymer electrodeposition system still suffers the aforedescribed disadvantages of film pitting. In addition, while the films electrodeposited from polyamic acids have excellent thermal stability at high temperatures, and would be useful, for example, as electrical insulation in stator core slots operating in 175.degree. C to 250.degree. C environments; the polyamic acid compositions have relatively low electrical ability (or low "throwing power") to deposit in the interior of hollow metal shapes. Thus, it is difficult to completely electrocoat the inside of complex shapes such as stator core slots using polyamic acid compositions.
Although most of the polymer electrophoretic deposition techniques applied in industry involve aqueous processes, a few organic systems, such as those shown in U.S. Pat. Nos. 3,450,655 and 3,463,714, have also been used. These systems would provide more pinhole free coatings. These systems have involved vinyl resins, epoxy resins, and carboxyl-containing polymers and copolymers such as polyacrylic acid, vinyl acetate/maleic acid copolymers, ethylene/itaconic acid copolymers and ethylene/maleic acid copolymers. However, these materials generally do not measure up to 175.degree. C to 250.degree. C requirements, and most of these resin systems do not possess the unique "throwing power" characteristics required to electrocoat the inside of complex shapes.
As regards the resin system, it has been found that a great number of variables exist in non-aqueous electrodeposition compositions, with respect to the ratio between resin polymer and solvent, and between solvents within the solvent system; and that each resin polymer used presents its own characteristic problems in its dilution or suspension and deposition. It has also been found that emulsions of the resin polymer give electrocoated films which are superior to those obtained from solutions of the resin polymer, but that emulsions require a very critical balance of component ingredients.
As regards "throwing power," according to Gauss' Law, the electrical charge on a hollow conductor is located on the surface, and the electric field within the conductor is non-existent. The electrocoating of pockets, corners and slots of a workpiece is limited by this phenomenon, commonly called the "Faraday Cage Effect." The ability of an electrodeposit to penetrate into the interior of hollow metal shapes depends, among other things, upon the coating composition component ratios described above, upon the particular electrical properties of the coating material, and upon the applied voltage.
What is needed then is an inexpensive, improved, non-aqueous electrodeposition method and composition. This composition should be a colloidal emulsion of a material having improved "throwing power," which will provide a coating with temperature capabilities in the 175.degree. C to 250.degree. C range; the composition being capable of electrical deposition in the interior of complex metal shapes, in thick uniform builds, without pinholes, upon a single electroapplication.